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Radiotracer method for simultaneous measurement of cation, anion and water transport through ion-exchange membranes
0969~8043/93
Appl. Radiur. hr. Vol. 44, No. IO/II, pp. 1399-1408, 1993 Rintcd in Great Britain. All rights -cd
$6.00 + 0.00
copyright0 1993PmgamonPm8 Ltd
Radiotracer Method for Simultaneous Measurement of Cation, Anion and Water Transport Through Ion-exchange Membranes ERIC W. SCHNEIDER
and MARK
W. VERBRUGGE
General Motors Research and Development Center, 30500 Mound Road, Box 9055, Warren,
MI 480904055, U.S.A. (Received IO February 1993)
A radiotracer method has been developed to characterize new fuel-cell membrane materials having high ionic conductance. Three radiotracers were employed simultaneously in cell studies and measured by a combination of liquid-scintillation and y-ray counting. This new method has been shown capable of measuring equilibrium concentrations and transport rates of sodium cations (representing counterions), chlorine anions (representing coions) and water molecules in ion-exchange membranes.
Introductios In this laboratory, sensitive radiotracer techniques have been developed for the characterization of ionexchange membranes. In particular, methods have been developed to measure: (i)
equilibrium concentrations of motile species within the membranes (Verbrugge and Hill, 1988); (ii) the diffusion rates of protons and bisulfate ions through the membranes (Verbrugge and Hill, 199Oa); and (iii) the transport of protons and water through the membranes as a function of current (Verbrugge and Hill, 1990b). These radiotracer techniques, which involve the simultaneous measurement of 3H and “?I, have been used to characterize a variety of promising membranes for fuel-cell applications (Verbrugge and Hill, 199Oc;Verbrugge et al., 199Oa; Guznan-Garcia et al., 1991). However, these techniques have two inherent disadvantages:
(0 electrochemical (conductivity) experiments must also be performed to provide enough experimental information for membrane analysis, the measurement of which is prone to greater error than the radiometric analysis; and (ii) it is difficult to analyze water transport because the ‘H exchanges with both water molecules and protons in the sulfuric acid electrolyte.
The lirst disadvantage is of particular concern in the evaluation of new membranes with high ionic conductance, in which case conductivity measurements have unacceptably high uncertainties. The goal of this work was to develop new analytical capabilities to eliminate these disadvantages. Previous work in this laboratory has shown that the ion-transport rates in ion-exchange membranes are directly related to the ionic radii (Verbrugge et al., 199Ob). Because of this, we can use other chemical species and still compare our results with our previous ‘H/% analyses by accounting for the different ionic radii. The work reported here utilizes three radionuclides: 3H, 2rNa and 36C1.By using these three radionuclides, we can eliminate the less precise electrochemical experiments for membrane analysis. In addition, by using nNa+ instead of protons for the positively charged species, we can accurately track water transport by monitoring ‘HOH transport. The small number of protons relative to the number of water molecules present will yield only a small amount of ‘H+. By using all three of the tracers simultaneously, we can characterize a membrane completely in one experiment, thereby eliminating uncertainties due to changes in conditions between successive tests. The Nafion- 117 poly(perlluorosulfonic acid) membrane is investigated in this study. This DuPont membrane has been studied extensively, and it is the most commonly employed polymer electrolyte membrane in fuel cells. Thus we may consider it as an ideal, model system in addition to being an electrolyte of practical importance.
1399
ERIC W. SCHNEIDERand
1400
Badiotracers have been used by others in the characterization of ionconducting membranes; 22Na, %, %I and 1251having been employed (Yeager et al., 1982; Herrera and Yeager, 1987). However, in all of these studies only a single tracer was employed at a time. Triple-label radiotracer studies have been employed in other applications, with 3H/14C/MC1 measurements being performed in plant physiology studies (Veen, 1974), and 3H/‘5Ca/36C1measurements being performed in studies of solute transport through soils (Nkedi-Kixza et al., 1985). These triplelabel combinations have involved p-emitting nuclides only. In our work, the use of 22Na, which also emits y radiation, provides the potential for greater flexibility in analytical measurement methods.
Principles Choosing a method of assay for multiple radiotracers in an ion-exchange membrane or in liquid electrolyte requires a review of the properties of each nuclide to be measured. Table 1 lists pertinent nuclear data for the three nuclides of interest in this study. Because ‘H and WI emit only j3 radiation, the method of choice for their measurement in solution is liquid scintillation (LS) counting. LS counting involves dissolving the sample directly into a liquid scintillator. Uncertainties associated with sample selfabsorption, attenuation by detector windows, and B backscattering are thereby avoided. For UNa, which emits both /I and y radiation, the potential exists for its measurement by either LS or y counting. In this work, two methods of analysis were evaluated for the determination of )H, ‘ZNa and %Cl within a single sample: (i)
simultaneous measurement of all species using LS counting only, and (ii) a combination of LS and y counting. Determination of three radiotracers by liquid scintillation counting The principles of triple-label LS counting are straightforward. Both of the commercial LS instrument manufacturers which offer software for the analysis of triple-labeled samples use virtually the same approach (Horrocks, 1982; Packard Instrument Company, 1988). The technique involves the selection of a region in the spectrum for each of the three nuclides. Each region is selected such that the intensity of the particular nuclide is optimized, as much as possible, with respect to the others. Standards (up to ten) are prepared for each individual nuclide, with Table 1. Nuclear properties of radionuclidca ion-exchange membrane stud& Nuclide
employedin
‘H
%a
WI
Half-life Q
12.33
2.60
3.01 x w
Decay mdc
IL -
511,127s
Max. fl cncrgy (kev) YmcmYckeLe\r)
.
8;4jy
-
MARKw. htLIRUC3CE each standard in a set having the same activity. Chemical quenching agents are added to the standards in varying amounts to determine the counting efficiency for each of the three nuclides in each of the three regions as a function of sample quench. The chemical quench of each sample is calculated automatically by the measurement of the Compton spectrtmr from an external y-ray source. This set of nine efficiency-quench curves represents the data base for the determination of triple-label activity levels from count-rate data. Another important system operation is the adjustment of the region windows with sample quench (automatic quench compensation or automatic efficiency control). This is needed to preserve reasonable counting efficiency in the “optimized” region as the energy spectrum shifts to lower energy with increased quench. For a sample with unknown levels of each tracer, its quench is first measured by the external source. Count rates in the three quench-compensated regions (C,, C, and Cc) are then determined. By using the nine quench curves, efficiency values in each region i (A, B and C) and for each nuclide j (1,2 and 3) can be obtained. Three simultaneous equations can then be written: C., = c,@, +
~2D2
+
~3D3,
Cs = E~,D, + ~~0~ +
+,D,,
Cc = a,@, +
ac34,
cc92
+
(1)
where C, is the count rate in region i, Q is the efficiency in region i of nuclide j, and Dj is the disintegration rate of nuclide j. A simple matrix algebra solution provides the disintegration rates, Dj, of the three nuclides. This method can provide reasonable analytical results when there is sutBcient energy separation between the different radionuclides and differences in quench among samples is minor (De Filippis, 1990). Discussions with Packard/ Canberra indicated that this approach may be suitable for our desired study of the 3H/2-%Ia/MC1tracers. Optima1 energy regions for each nuclide were also suggested (Passe, 1990). Determination of three radiotracers by a combination of liquid scintillation and y counting Of the three tracers used in this study, only “Na emits y radiation. It is therefore possible to determine its disintegration rate accurately by y counting with a NaI(Tl) scintillation crystal. The presence of ‘H and %Cl do not produce a significant interference to this determination. The ‘H and =Cl activities can be determined by conventional dual-label LS counting, using external quench measurement and automatic efficiency control, as described above. Interferences from the presence of %a can be corrected by use of a set of reference standards with known ratios of ‘H, =Na and 36C1.This method should provide reasonable results if the corrections for =Na can be kept
Radiotracers-ion-exchange membrane studies small by restricting the =Na concentration value relative to the other species.
to a low
Experimental Method development Preparation and assay of stock solutions and standards. The three radiotracers, purchased from DuPont New England Nuclear, were diluted with deionized water to prepare stock solutions. Stock solutions were diluted further to prepare standard solutions with radionuclide concentrations similar to those estimated for the “hot” and “cold” reservoirs during ion-transport studies. (The “hot” reservoir is the side initially spiked with radiotracers, and the “cold” reservoir is separated from the hot reservoir by the ion-conducting membrane and contains no initial radiotracer spike.) The hot-side standard (HSS) solutions were prepared by a 200 x dilution of the stock solutions into 1 M NaCl. The cold-side standard (CSS) solutions were prepared by a further dilution of the HSS solutions with deionized water: 200 x for zNa and “Cl, and 1000 x for ‘H. Assays of the ‘H solutions were performed by LS counting using a Packard Tri-Carb CA2000 Liquid Scintillation Analyzer. Efficiency and quench corrections for ‘H were performed using a set of quench standards (Packard, Inc.). Sample quench was calculated from the Compton spectrum induced into each sample by an external ia3Ba source, and reported as the Quench Indicating Parameter Number (QIP No.) for each sample. System performance was monitored with a reference standard (Packard, Inc.). Assays of %Cl solutions were performed by the efficiency tracing method (Ishikawa, 1984; Packard Instrument Company, 1988b). Assays of the 22Na solutions were performed by use of a 76 x 76 mm NaI(T1) well counter and a 22Na reference standard (Amersham, Inc.). Properties of the stock and standard solutions and results of assays are listed in Table 2. Preparation and assay of quench standards and triple-label standardr. Quench standards of the three nuclides were prepared for use in dual-label and triple-label LS counting. For each nuclide, ten IOO-PL samples were drawn from the respective HSS solution using Kirk-type micropipets and transferred into 15-mL aliquots of Instagel-XF liquid scintillation cocktail (Packard, Inc.), in LS counting vials. Radioactivity levels were measured to verify uniformity of activity for each sample set. Additions of different amounts of CH,NO, (O-300 ).tL) were then Table 2. Radiotracer stock and standard solutions Stock SOlUtiOll
Nuclide za* Mel
Chemical form
activity ( pCi/mL)
HSS activity (DPM/p L)
css activity (DPM/mL)
Nflcl Hz0 HCI
400 100 100
3420 1073 960
5120 3410 4780
1401
made to each sample in the set to provide a range of chemical quenching. The samples were counted again by LS spectroscopy, with an external y-ray source being used to determine the quench of each sample. A triple-label standard set was prepared by pipeting various quantities of the three radiotracers from the HSS solutions into 15-mL aliquots of Instagel-XF LS cocktail. All possible combinations of 0, 50 and 300 ,uL of uNa and 36Cl, and 0, 15 and 50 PL of ‘H were prepared, resulting in a set of 27 HSS triple-label standards. A similar set of 27 CSS triple-label standards was also prepared. Amounts of the HSS and CSS solutions were selected to span the range of concentrations expected in the hot and cold reservoirs, respectively, during cell studies. Both sets were assayed by LS and y counting. Liquid scintillation and y counting The LS counting system was calibrated for dualand triple-label counting by using the quenchstandard set for each appropriate nuclide. For the dual-label analysis, energy windows were set O-14 keV for 3H and 14-710 keV for 36C1.For the triple-label analysis, energy windows were set at O-12 keV for 3H, 12-200 keV for =Na, and 200-7 10 keV for 36C1.Counting times for the HSS set were selected to be the minimum of 5 min or a 0.5% uncertainty in the count of each region. For the CSS set and for membrane analyses, counting times were the minimum of 60 min or a 0.5% uncertainty in the count of each region. The y-ray analyses were performed using the 76 x 76-mm NaI(T1) well counter, with associated electronics and timer/counter (EG&G Ortec). Counting times ranged from 200 to 2000 s, depending on the activity of the sample. Ion-transport studies of membranes Films of various commercial ion-conducting membranes were conditioned and assembled into a radiotracer cell, providing the separation between the two 50-mL reservoirs. After the addition of 1 M NaCl electrolyte to each reservoir, the three radiotracers were added to the “hot” side. Various concentrations of each radionuclide were added to the hot side during the studies in an effort to obtain desired ratios of the three species in the cold reservoir during transport and diffusion studies. Best results were obtained with initial amounts of ca 4000, 250 and 20,000 dpm/pL for 3H, 22Na and 36C1,respectively. Following the addition of the radiotracers, the cell was operated at current densities of 10, 100 and 300 A/m2. During each interval of constant current, samples of 200-PL volume were extracted from the cold side at equal time periods and transferred to 15-mL volumes of LS cocktail. In addition, the cell was monitored for an extended period of time with no current being passed to measure diffusion rates of the ions and water across the membrane. Periodic samples of the hot-side reservoir were also obtained
ERICW. SCHNE~OW and MARKW. VERLUUJGGE
1402
during the studies. Specific results for the variety of membranes under investigation will be the subject of a future report. Equilibrium studies were also performed to determine the concentrations of mobile species contained within the membranes. This was accomplished by allowing conditioned membranes to equilibrate in a radiotracer solution identical to that of the hot-side reservoir. The membranes were subsequently removed, blotted dry of any excess solution, and equilibrated in 15-mL volumes of LS cocktail before LS and y counting. Redts
and Discussion
Liquid scintillation analyzer eJ%ziency calibration For the triple-label LS analyses, efficiencies were determined by using the quench standards for each of the three nuclides. Spectra of the three sample types, each containing only one of the three tracers, are shown in Fig. 1. Also denoted are the three counting regions selected for triple-label LS analysis. The settings have been shifted lower from the preset values by automatic quench compensation. Resultant efficiency-quench plots for each nuclide in each counting window are reported in Fig. 2. For the dual-label LS analyses, efficiencies were determined in a similar fashion to the triple-label case by using the ‘H and WI quench standards. At nominal quench values for electrolyte samples in this study, the dual-label LS counting efficiency was 53%
for 3H and 95% for 36C1.The interference of each nuclide in the counting region of the other was minor, and never exceeded 3%. Dual-label counting efficiencies for ‘H and ?l were somewhat higher than for the triple-label counting because of the less restrictive energy regions. Assay of hot-side and cold-side triple-label standards The HSS set was counted by both the triple-label LS procedure and dual-label LS/y -ray counting. Results were used to determine which of the two analytical approaches provided the greatest accuracy over the wide range of concentrations evaluated. It can be seen from the spectra in Fig. 1 that virtually all of the intensity from 3H occurs at very low fl energy (Region A). An accurate ‘H assay should therefore be possible without much interference from 22Na or 36C1.The 22Na and 36C1spectra, however, are quite similar, and there will be significant interference whatever regions are selected. Ranges for Regions B and C were chosen for optimal sensitivity to the small differences between the two spectra. Results of the triple-label analysis of the HSS set are reported in Table 3. There is little difficulty in the determination of 3H, regardless of how much 22Na or 36C1is present. However, significant underestimation of 22Na occurs at a variety of ‘H and ‘6C1 concentrations (e.g. Nos 8, 17). Also, significant overestimation of %Cl occurs at low and moderate values of 22Na (e.g. Nos 4, 26). Because there is no other means to
0000
0 g
4000
6 8 z 0 d c g 0
4000
0
200
200
400
600
Energy, keV Fig. 1. Overlaid individual liquid scintillation spectra of ‘H, 22Naand 36C1.Quench-shifted energy regions for the triple-label analyses are also shown. (Spectra have been nonnalii for display purposes.)
6
Radiotracm-ionclrchange
membrane studies
1403
60 l Region
A
(a>
3H
o
o-e-o-q-0 --500
K! n
F
.-9, 0
l Region
A
0 Region
B
l Region
C
-0-m
tF
is 40-
cl
80
60
l Region
A
D Region
B
l Region
C
*-o-o-o-.
40
m-m-B-m-m
Sample Quench, QIP # Fig. 2. Counting e&iency of the LS spectrometer as a function of chmical quench (Quench Indicating Paramcm, QIP No.) for: (a) ‘H, (b) %Ja and (c) %.
ARI 44-10/11-K
ERIC w.
1404
SCHNEIDER
and
Table 3. Analvsis of hot-side standards bv triole-label LS countinn Stock solution added OtL) No.
‘H
1
0 0 0 0 0 0 0 0 0
2 3 4 5 6 7 8 9 10 11 12 I3 14 15 16 17 18 19 20 21 22 23 24 25 26 27
%I
0
0
so
0
WI 0 0 0
300
so
:,
50 50
15 15 15 15 15 15 15 15 15 50 50 50 50 50 50 50 50 50
‘H
0 0 0 0 0 1 0 0 2 0 0
50” 300 0 50 300 0 50 300 0
300 300 300 0 0 0 50 So 50 300 300 300 0 0 0 so 50 50 300 300 300
3:
-1 -2 -2
0 3z 0 SO 300 0 50 300 0 50 300
1 0 I -0 -0 0 -2 -2 -4
Standard deviition
9 -1 -8 20 23 -7 2 0 0 8 5 2 26 40 -I5 4 0 0 5 8 5 37 47 -5
1.2
w.
bUlRUGGE
Table 5. Analysis of hot-side standards counting
0
-2 -30 0 -5 -35 0 -26 -31 0 -7 -39 0 -11 -41 0 -39 -22 0 -3 -33 0 -13 -44 0 -42 -34
14.3
16.7
correct for these deviations, the triple-label LS method for 3H/22Na/36Cl,with the Packard DPM123 software as employed, provides only semi-quantitative results for the range of tracer concentrations of interest in this study, and, more generally, for the characterization of most membrane electrolytes. Table 4. Analysis of hot-side standards before the addition of =Na usiw dual-label LS countina
No.
‘H
1 2 3 4 S 6 7 8 9 10 11 12 13 14 IS 16 17 18 19 20 21 22 23 24 25 26 27
0 0 0 0 0 0 8 0 15 15 15 15 15 15 15 15 I5 50 50 50 50 50 50 so 50 50
WI
“Na
0 0 0 50 50 50 300 300 300 0 0 0 50 50 SO 300
0 50 300 0 50 300 0 50 300 0 SO 300 0 50 300 0
:;
3:
0 0 0 50 50 50 300 300 300
0 50 300 0 50 300 0 50 300
No.
‘H
MC1
*Na
1 2 3 4 5 6 7 8 9 10 11 12 13 14 1s 16
0 0 0 0 0 0 0 0 0 15 15 I5 IS 15 15 1s
0 0 0 50
0 0 0 0 0 0 0 0 0 0 0 0 0 0 :
0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.2 0.3 0.3 0.5 0.1 0.6 0.6 0.1 0.5
0.0 0.0 0.0 -0.1 0.8 0.2 -1.7 -3.7 -1.0 0.0 0.0 0.0 -0.4 -0.2 0.4 3.1
17 18 19 20 21 22 23 24 25 26 27
15 50 50 50 50 50 50 SO 50 50
08
OS 0.4 0.8 0.3 0.8 0.4 0.7 0.7 -0.1 -0.6 -0.2
:.: 0:o 0.0 0.0 0.3 0.8 -0.2 2.3 2.6 1.9
0.3
1.5
z 300 300 300 0 8 SO SO SO 300 300 0 0 0 50 z 300 300 300 Standard deviation
0 0 0 0 0 0 0 0
WI
‘H
‘%I
“Na
0.0 0.2 1.4 0.0 0.2 1.4 0.2 0.2 1.4 0.4 0.6 0.1 1.0 1.0 0.2 0.5 0.5 -0.4 1.2 0.7 0.6 0.8 0.9 -0.8 -0.3 -0.4 -2.0
-0.0 -0.2 7.2 -0.2 0.7 7.6 -3.8 -2.1 8.4 -0.0 0.2 8.2 -0.3 0.2 7.7 2.2 2.3 7.0 0.0 -0.4 ::9 1.3 7.1 0.2 1.5 9.0
0.1 1.2 -3.5 0.1 0.8 -4.1 0.3 1.6 -3.2 0.0 0.4 -3.4 0.0 0.4 -2.8 0.4 0.5 -2.8 -0.0 1.0 -4.1 0.1 0.7 -2.9 0.4 -0.0 -2.5
0.7
3.8
1.8
Results of the dual-label LS and y-ray analyses of the HSS set are reported in Tables 4 and 5. Table 4 shows counting results of the standards before addition of the *%a radiotracer. These values indicate
Table 6. Analysis of cold-side standards countinn
by dual-label LS and y
Stock solution added 01L)
Error @L) ‘H
Error (JIL)
“Na
Standard deviation
Stock solution added (pL)
by dual-label LS and y
Stock solution added OCL)
Error @L)
*Na
0
MARK
No.
IH
: 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
0 0 0 0 0 0 0 0 100 100 100 100 100 100 100 100 100 300 300 300 300 300 300
f: :: 27
300
Error 01 L)
WI
*Na
‘H
WI
=Na
0
100 0 300
-0.0 0.7 1.4 1.5 1.2 2.1 0.3 0.3 2.6 -2.0 0.6 -1.8 2.0 0.3 -0.0 -0.3 -0.3 0.3 -0.6 -2.9 -2.0 -1.5 -2.3 -6.7 -3.2 -3.9 -5.3
-3.1 0.2 0.4 -1.9 1.5 -0.2 -1.4 -2.9 -0.8 -0.7 0.1 1.9 2.0 -1.5 2.1 -2.4 -3.0 -0.9 0.5 -1.2 -4.0 0.9 -1.3 2.9 0.2 0.3 2.2
-0.3 3.4 -2.6 1.4 -0.6 0.2 0.2 -0.0 -1.3 0.8 -0.2 -0.4 -1.8 0.5 -1.7 -0.4 -0.9 1.6 -0.6 1.9 2.7 -0.6 0.9 -0.8 0.2 -2.7 0.0
2.2
1.7
1.4
0
100 100 100 300 300 300 0 0 0 100 100 100 300 300 300 0 0 0 100 100 :$ 300 300
0
loo 300 0 100 300 0 100 300 0 100 300 0 100 300 0 100 300 0 100 300 0 100 300
Standard deviation
Radiotracers-ion-exchange Table 7. Equilibrium uptake study of N&h-l tripk-label radiotracctx
1 2 3
‘H
‘%I
16360 17620 13180
11860 15220 11950
%I
‘H
%I
%Ja
8910 9340 8780
5.1 5.5 4.1
1.0 1.2 1.0
35.2 36.9 34.7
1405
Tabk 8. Initial activity concentrations of each txdiotraw in Nafioo-117 transport studies
17 membranes using Equiv. vol. of clcctrolytc (p L)
Net activity (DPM) No.
membrane studies
Activity amccotfation @PW L) Cell No. 2 3 4 5
that the dual-label approach for simultaneous determination of 3H and “Cl is accurate over a wide concentration range of either tracer. Because y-ray measurements are not alEcted by the presence of ‘H or “Cl, the determination of “Na can be performed accurately and independently of the other tracers. After the uNa was added to the HSS set, y-ray measurements were made and dual-label LS measurements were performed again. Correction factors were then determined to adjust for interferences of the ‘H and WI LS measurements by uNa. The resultant factors, when multiplied by the **Na count rate and added to the measured sample activity level, provide the appropriate corrections to the LS results. Table 5 shows analytical results for the HSS standards using this approach. The correction factors were +0.03 for )H, and - 1.61 for %I. In general, error values are reasonably low across the concentration ranges of each tracer. The only area of concern is the case of samples with high levels of “Na, in which case the “YJl values are overestimated (e.g. Nos 3,6). This error could have been reduced by the selection of a different correction factor for YJl, at the expense of elevating errors at other concentrations. It was decided to employ the correction
‘H
WI
nNa
960 4100 4100 0
3800 20,200 20,200 8100
;o” 0 0
factors as listed and design the experiment to keep the **Na/Yl ratio low enough to avoid the area of concern. As a check of LS/y-ray counting method, the CSS sample set was prepared. The CSS set was similar to the HSS set, but had concentrations of activity at levels expected in the cold reservoir during the iontransport studies. Analytical results for the CSS set are reported in Table 6. The correction factors for =Na obtained from the HSS set were employed in the analysis. Again, reasonable results are obtained for each species over a wide concentration range. Because of the demonstrated accuracy of dual-label LS/y-ray counting method for the HSS and CSS sets, this analytical procedure was selected for the triplelabel radiotracer measurements in all subsequent ion-transport membrane studies. Equilibrium studies Representative results from equilibrium uptake studies of NaGon- 17 membranes are listed in Table 7. Displayed are net disintegration rates of each species as well as the corresponding equivalent volume of electrolyte, which is obtained from the
0
Time, min Fig. 3. Relative concentration of 3H in the cold reservoir as a function of time for four different currents.
EJUCW. ScHNmDER and MARKW. VERBRUOGE
1406
‘j
1.0
C g 0 8
OS .
0.0 0
Time, min Fig. 4. Relative concentration of %I in the cold reservoir as a function of time for four different currents.
ratio of the membrane uptake activity to the activity per microliter of the equilibration solution. Repeatable results have been obtained for all types of membranes studied, and will be reported separately. Count rates for the Na6on membranes were in the same range as a mixture of 5 nL 3H, 14 PL of Yl, and 8 nL %a from the HSS solutions. With reference to Table 5, this tracer range should yield accuracies for all species within about f5%.
Transport studies Transport properties of Nafion- 117 membranes were obtained from a series of four cell studies (Cells 2-5). Initial hot-side concentrations of the tracers in these cells are listed in Table 8. When compared to initial concentrations of ‘H, concentrations of %Cl were elevated to compensate for model predictions (Verbrugge and Hill, 199Oa, b, c) of its low relative transport rate. Likewise, initial concentrations of
0 0
200
400
600
800
1000
1200
1400
14
Time, min Fig. 5. Relative concentration of zzNa in the cold reservoir as a function of time for four different currents.
Radiotracers-ion-exchange
0.0 0
200
400
600
1407
membrane studies
800
1000
1200
1400
16
Time, mln Fig. 6. Relative concentration of 36C1in the cold reservoir as a function of time for four clitIerentcurrents. (Current passage is reversed from the previous cells, with the cold-side electrode being the anode.)
22Na were reduced to compensate for predictions of its high relative transport rate. Cells 2 and 3 employed all three tracers simultaneously. To verify results obtained in the three-tracer studies, Cell 4 employed only ‘H and 36C1,and Cell 5 employed only W. Results from the transport studies of Nafion- 117 are summarized in Figs 3-6. Each plots the ratio of the concentration of the radiotracer in the cold side relative to its concentration in the hot side. All cells were operated for a period of 1440min. Current Regions l-4 labeled on each plot represent current densities of 10, 100, 300 and 0 A/m*, respectively. In each case it is seen that there is a linear increase of activity in the cold reservoir with time at a given current. The slopes of the lines in these plots are used to acquire transport properties. Slight differences in the slopes for equivalent experiments are likely due to differences in the membrane used for each experiment, rather than errors introduced in the radiometric analysis. Other than having different absolute concentrations of the radiotracers, Cells 2 and 3 runs were duplicate experiments. Figures 3, 4 and 5 indicate excellent reproducibility for all species at all currents. Cell 4 was an additional repeat of conditions, but without the *Na radiotracer. Again, Figs 3 and 4 show excellent reproducibility. The agreement between Cell 4 and the previous two runs indicates that the inclusion of the 22Na did not interfere with either the ‘H or %Cl measurements. Results from Cells 2, 3 and 4 all indicated that virtually none of the %Cl radiotracer was transported from the hot side to the cold side when current densities were at or above 100 A/m*. To determine
whether there was a net flow of Cl- in the opposite direction during these conditions, an additional experiment was performed. In this case, only the Wl tracer was employed, and the direction of current passage was reversed so that the cold-side electrode was the anode. Results, shown in Fig. 6, do indeed indicate that the net flow of Cl- is in the opposite direction during high-current conditions. Diffusion rates (Current Region 4) are similar to those measured during the prior three cell runs. Results from these four cell studies indicate that the transport of the three tracers can be tracked simultaneously using the LS/y-ray counting method, and that there are no significant interferences between any of the species. A single experiment can determine the transport rates as well as the diffusion rates of all three species in a given direction as a function of current density. conclusions A triple-label radiotracer method has been developed to characterize ion-exchange membranes for fuel-cell applications. By utilizing ‘HOH for water transport, uNa+ for cation transport, and 3sClfor anion transport, a membrane can be characterized completely and accurately in a single set of measurements. Combinations of the three radiotracers, spanning a wide range of relative concentrations, were used to evaluate various nuclear analytical methods for analysis of electrolyte and membrane samples. The simultaneous determination of all species by triplelabel liquid scintillation counting was found to provide only semi-quantitative results. However, a
Eiuc W. &XNEwsa and
1408
combination of liquid scintittation and y-ray counting provided accurate results over the concentration ranges of interest in this study. To demonstrate the capabilities of this new anatyticat technique, Nation-t t 7 ion-exchange membranes were evaIt&ed for equilibrium conc&trations and transport rates of ail mobile species. Results show that the measurements are reproducible and there are no siunificant interferences between anv of the species. By using the newly developed three-tracer method, we have shown that the transport phenomena of ion-exchange membrane materials can he XCUratetY characterized, which attows US to Predict their utility in fuel-cell applications.
References De Filippis P. (1990) Activity analysis in liquid scintillation coun&g: a. techhical GmpaGtive summary-Part I. Radioact. Radio&m. FdL 22.
Guzman-Garcia A., Verbrugge M. W., Schneider E. W. and Pintauro P. N. (1991)Analysis of radiation-grafted membranes for fuel-cell electrolytes, Research Publication GMR-7225. Presented at tlte Spring Electrochemical Society Meeting, Washington, D.C., May, 1991,Abstract 7%. J. Appi. Electrochem. 22. 204. Herrera A. -and Yeager H. L. (1987) Halide and sulfate ion diffusion in nagon membranes. J. Electrochem. Sot. 134, 2446.
Horrocks D. L. (1982) Triple label DPM determinations by liquid scintillation counting. Trans. Am. Nucl. Sot. 43,39. Ishikawa H., Takiue M. and Aburai T. (1984) Radioassay bv an elEciencv tracing teclmiaue usina a liauid scintill&on counter.-Znr. J. >ppl. R&at. Z&t. 3$‘463. Nkedi-Kizza P.. Jackson R. K. and MacIntvre J. L. (1985) Isotopic triple labeling in miscible displacement studies~ Soil Sci. Sot. Am. J. 49(3), 780 (1985).
MARK
w.
bRBIlUGGE
Packard Instrument Company (1988a) DPM l-2-3 Option, Operation Manual, Packard/Canberra Industries, Downers Grove, IL. Packard Instrument Company (1988b) Tri-Carb Liquid Scintillation Analyzer Model 22OOCA, Operation Manual, Appendix H, Packard/Canberra Industries, Downers Grove, IL. Pass0 C. (1990) Personal Communication, Packard/ Canberra Industries Marketing Applications Group. Veen H. (1974) Data processing of triple labelled liquid scintillation sam~ks. Inl. J. ad. Radial. Zsot. 2?i. 355. Verbrugge M. W. -&rdHill R. F.-(1988) Experimental and theoretical investigation of pertluorosulfonic acid membranes equilibrated with aqueous sulfuric acid solutions. J. Phys. Chem. 92, 23, 6778.
Verbruaae M. W. and Hill R. F. (199OalIon and solvent transGrt in ion-exchange membranes.‘II. A radiotracer study of the sulfuric-acid, Nalion-117 system. J. Electrochem. Sot. 137. 893.
Verbrugge M. W. .and Hill R. F. (1990b) Transport nhenomena in nerlIuorosulfonic acid membranes during the passage’ of current. J. Electrochem. Sot. 137, 1131. Verbruaae M. W. and Hill R. F. (199Oc) Analysis of prosing perlluorosulfonic acid membranes for fuel-cell electrolytes. J. Electrochem. Sot. 137, 3370. Verbrugge M. W., Hill R. F., Guzman A. and Pintauro P. N. (199Oa)The effect of ionic radii on the absorption and transport characteristics of ion-exchange membranes, Research Publication GMR-6474. Presented at the Spring Electrochemical Society Meeting, Los Angeles, CA, May, 1989, Abstract 626. AZChE J. 36, 1061. Verbrngge M. W., Hill R. F. and Schneider E. W. (199Ob) Composite poly (tetralIuoroetbylene)/perlluorosulfonic acid membranes for fuel-cell applications, Research Publication GMR-7023. Presented at the Annual Meeting of the American Institute of Chemical Enaineers, Chicago, IL, November, 1990, Paper No. 198b. AZChE J. 38, $3. Yeaaer H. L.. O’Dell B. and Twardowski Z. (1982) Transp&t properties of Nation membranes in conc&trated solution environments. J. Elecfrochem. Sot. 129, 85.
Appl. Radiur. hr. Vol. 44, No. IO/II, pp. 1399-1408, 1993 Rintcd in Great Britain. All rights -cd
$6.00 + 0.00
copyright0 1993PmgamonPm8 Ltd
Radiotracer Method for Simultaneous Measurement of Cation, Anion and Water Transport Through Ion-exchange Membranes ERIC W. SCHNEIDER
and MARK
W. VERBRUGGE
General Motors Research and Development Center, 30500 Mound Road, Box 9055, Warren,
MI 480904055, U.S.A. (Received IO February 1993)
A radiotracer method has been developed to characterize new fuel-cell membrane materials having high ionic conductance. Three radiotracers were employed simultaneously in cell studies and measured by a combination of liquid-scintillation and y-ray counting. This new method has been shown capable of measuring equilibrium concentrations and transport rates of sodium cations (representing counterions), chlorine anions (representing coions) and water molecules in ion-exchange membranes.
Introductios In this laboratory, sensitive radiotracer techniques have been developed for the characterization of ionexchange membranes. In particular, methods have been developed to measure: (i)
equilibrium concentrations of motile species within the membranes (Verbrugge and Hill, 1988); (ii) the diffusion rates of protons and bisulfate ions through the membranes (Verbrugge and Hill, 199Oa); and (iii) the transport of protons and water through the membranes as a function of current (Verbrugge and Hill, 1990b). These radiotracer techniques, which involve the simultaneous measurement of 3H and “?I, have been used to characterize a variety of promising membranes for fuel-cell applications (Verbrugge and Hill, 199Oc;Verbrugge et al., 199Oa; Guznan-Garcia et al., 1991). However, these techniques have two inherent disadvantages:
(0 electrochemical (conductivity) experiments must also be performed to provide enough experimental information for membrane analysis, the measurement of which is prone to greater error than the radiometric analysis; and (ii) it is difficult to analyze water transport because the ‘H exchanges with both water molecules and protons in the sulfuric acid electrolyte.
The lirst disadvantage is of particular concern in the evaluation of new membranes with high ionic conductance, in which case conductivity measurements have unacceptably high uncertainties. The goal of this work was to develop new analytical capabilities to eliminate these disadvantages. Previous work in this laboratory has shown that the ion-transport rates in ion-exchange membranes are directly related to the ionic radii (Verbrugge et al., 199Ob). Because of this, we can use other chemical species and still compare our results with our previous ‘H/% analyses by accounting for the different ionic radii. The work reported here utilizes three radionuclides: 3H, 2rNa and 36C1.By using these three radionuclides, we can eliminate the less precise electrochemical experiments for membrane analysis. In addition, by using nNa+ instead of protons for the positively charged species, we can accurately track water transport by monitoring ‘HOH transport. The small number of protons relative to the number of water molecules present will yield only a small amount of ‘H+. By using all three of the tracers simultaneously, we can characterize a membrane completely in one experiment, thereby eliminating uncertainties due to changes in conditions between successive tests. The Nafion- 117 poly(perlluorosulfonic acid) membrane is investigated in this study. This DuPont membrane has been studied extensively, and it is the most commonly employed polymer electrolyte membrane in fuel cells. Thus we may consider it as an ideal, model system in addition to being an electrolyte of practical importance.
1399
ERIC W. SCHNEIDERand
1400
Badiotracers have been used by others in the characterization of ionconducting membranes; 22Na, %, %I and 1251having been employed (Yeager et al., 1982; Herrera and Yeager, 1987). However, in all of these studies only a single tracer was employed at a time. Triple-label radiotracer studies have been employed in other applications, with 3H/14C/MC1 measurements being performed in plant physiology studies (Veen, 1974), and 3H/‘5Ca/36C1measurements being performed in studies of solute transport through soils (Nkedi-Kixza et al., 1985). These triplelabel combinations have involved p-emitting nuclides only. In our work, the use of 22Na, which also emits y radiation, provides the potential for greater flexibility in analytical measurement methods.
Principles Choosing a method of assay for multiple radiotracers in an ion-exchange membrane or in liquid electrolyte requires a review of the properties of each nuclide to be measured. Table 1 lists pertinent nuclear data for the three nuclides of interest in this study. Because ‘H and WI emit only j3 radiation, the method of choice for their measurement in solution is liquid scintillation (LS) counting. LS counting involves dissolving the sample directly into a liquid scintillator. Uncertainties associated with sample selfabsorption, attenuation by detector windows, and B backscattering are thereby avoided. For UNa, which emits both /I and y radiation, the potential exists for its measurement by either LS or y counting. In this work, two methods of analysis were evaluated for the determination of )H, ‘ZNa and %Cl within a single sample: (i)
simultaneous measurement of all species using LS counting only, and (ii) a combination of LS and y counting. Determination of three radiotracers by liquid scintillation counting The principles of triple-label LS counting are straightforward. Both of the commercial LS instrument manufacturers which offer software for the analysis of triple-labeled samples use virtually the same approach (Horrocks, 1982; Packard Instrument Company, 1988). The technique involves the selection of a region in the spectrum for each of the three nuclides. Each region is selected such that the intensity of the particular nuclide is optimized, as much as possible, with respect to the others. Standards (up to ten) are prepared for each individual nuclide, with Table 1. Nuclear properties of radionuclidca ion-exchange membrane stud& Nuclide
employedin
‘H
%a
WI
Half-life Q
12.33
2.60
3.01 x w
Decay mdc
IL -
511,127s
Max. fl cncrgy (kev) YmcmYckeLe\r)
.
8;4jy
-
MARKw. htLIRUC3CE each standard in a set having the same activity. Chemical quenching agents are added to the standards in varying amounts to determine the counting efficiency for each of the three nuclides in each of the three regions as a function of sample quench. The chemical quench of each sample is calculated automatically by the measurement of the Compton spectrtmr from an external y-ray source. This set of nine efficiency-quench curves represents the data base for the determination of triple-label activity levels from count-rate data. Another important system operation is the adjustment of the region windows with sample quench (automatic quench compensation or automatic efficiency control). This is needed to preserve reasonable counting efficiency in the “optimized” region as the energy spectrum shifts to lower energy with increased quench. For a sample with unknown levels of each tracer, its quench is first measured by the external source. Count rates in the three quench-compensated regions (C,, C, and Cc) are then determined. By using the nine quench curves, efficiency values in each region i (A, B and C) and for each nuclide j (1,2 and 3) can be obtained. Three simultaneous equations can then be written: C., = c,@, +
~2D2
+
~3D3,
Cs = E~,D, + ~~0~ +
+,D,,
Cc = a,@, +
ac34,
cc92
+
(1)
where C, is the count rate in region i, Q is the efficiency in region i of nuclide j, and Dj is the disintegration rate of nuclide j. A simple matrix algebra solution provides the disintegration rates, Dj, of the three nuclides. This method can provide reasonable analytical results when there is sutBcient energy separation between the different radionuclides and differences in quench among samples is minor (De Filippis, 1990). Discussions with Packard/ Canberra indicated that this approach may be suitable for our desired study of the 3H/2-%Ia/MC1tracers. Optima1 energy regions for each nuclide were also suggested (Passe, 1990). Determination of three radiotracers by a combination of liquid scintillation and y counting Of the three tracers used in this study, only “Na emits y radiation. It is therefore possible to determine its disintegration rate accurately by y counting with a NaI(Tl) scintillation crystal. The presence of ‘H and %Cl do not produce a significant interference to this determination. The ‘H and =Cl activities can be determined by conventional dual-label LS counting, using external quench measurement and automatic efficiency control, as described above. Interferences from the presence of %a can be corrected by use of a set of reference standards with known ratios of ‘H, =Na and 36C1.This method should provide reasonable results if the corrections for =Na can be kept
Radiotracers-ion-exchange membrane studies small by restricting the =Na concentration value relative to the other species.
to a low
Experimental Method development Preparation and assay of stock solutions and standards. The three radiotracers, purchased from DuPont New England Nuclear, were diluted with deionized water to prepare stock solutions. Stock solutions were diluted further to prepare standard solutions with radionuclide concentrations similar to those estimated for the “hot” and “cold” reservoirs during ion-transport studies. (The “hot” reservoir is the side initially spiked with radiotracers, and the “cold” reservoir is separated from the hot reservoir by the ion-conducting membrane and contains no initial radiotracer spike.) The hot-side standard (HSS) solutions were prepared by a 200 x dilution of the stock solutions into 1 M NaCl. The cold-side standard (CSS) solutions were prepared by a further dilution of the HSS solutions with deionized water: 200 x for zNa and “Cl, and 1000 x for ‘H. Assays of the ‘H solutions were performed by LS counting using a Packard Tri-Carb CA2000 Liquid Scintillation Analyzer. Efficiency and quench corrections for ‘H were performed using a set of quench standards (Packard, Inc.). Sample quench was calculated from the Compton spectrum induced into each sample by an external ia3Ba source, and reported as the Quench Indicating Parameter Number (QIP No.) for each sample. System performance was monitored with a reference standard (Packard, Inc.). Assays of %Cl solutions were performed by the efficiency tracing method (Ishikawa, 1984; Packard Instrument Company, 1988b). Assays of the 22Na solutions were performed by use of a 76 x 76 mm NaI(T1) well counter and a 22Na reference standard (Amersham, Inc.). Properties of the stock and standard solutions and results of assays are listed in Table 2. Preparation and assay of quench standards and triple-label standardr. Quench standards of the three nuclides were prepared for use in dual-label and triple-label LS counting. For each nuclide, ten IOO-PL samples were drawn from the respective HSS solution using Kirk-type micropipets and transferred into 15-mL aliquots of Instagel-XF liquid scintillation cocktail (Packard, Inc.), in LS counting vials. Radioactivity levels were measured to verify uniformity of activity for each sample set. Additions of different amounts of CH,NO, (O-300 ).tL) were then Table 2. Radiotracer stock and standard solutions Stock SOlUtiOll
Nuclide za* Mel
Chemical form
activity ( pCi/mL)
HSS activity (DPM/p L)
css activity (DPM/mL)
Nflcl Hz0 HCI
400 100 100
3420 1073 960
5120 3410 4780
1401
made to each sample in the set to provide a range of chemical quenching. The samples were counted again by LS spectroscopy, with an external y-ray source being used to determine the quench of each sample. A triple-label standard set was prepared by pipeting various quantities of the three radiotracers from the HSS solutions into 15-mL aliquots of Instagel-XF LS cocktail. All possible combinations of 0, 50 and 300 ,uL of uNa and 36Cl, and 0, 15 and 50 PL of ‘H were prepared, resulting in a set of 27 HSS triple-label standards. A similar set of 27 CSS triple-label standards was also prepared. Amounts of the HSS and CSS solutions were selected to span the range of concentrations expected in the hot and cold reservoirs, respectively, during cell studies. Both sets were assayed by LS and y counting. Liquid scintillation and y counting The LS counting system was calibrated for dualand triple-label counting by using the quenchstandard set for each appropriate nuclide. For the dual-label analysis, energy windows were set O-14 keV for 3H and 14-710 keV for 36C1.For the triple-label analysis, energy windows were set at O-12 keV for 3H, 12-200 keV for =Na, and 200-7 10 keV for 36C1.Counting times for the HSS set were selected to be the minimum of 5 min or a 0.5% uncertainty in the count of each region. For the CSS set and for membrane analyses, counting times were the minimum of 60 min or a 0.5% uncertainty in the count of each region. The y-ray analyses were performed using the 76 x 76-mm NaI(T1) well counter, with associated electronics and timer/counter (EG&G Ortec). Counting times ranged from 200 to 2000 s, depending on the activity of the sample. Ion-transport studies of membranes Films of various commercial ion-conducting membranes were conditioned and assembled into a radiotracer cell, providing the separation between the two 50-mL reservoirs. After the addition of 1 M NaCl electrolyte to each reservoir, the three radiotracers were added to the “hot” side. Various concentrations of each radionuclide were added to the hot side during the studies in an effort to obtain desired ratios of the three species in the cold reservoir during transport and diffusion studies. Best results were obtained with initial amounts of ca 4000, 250 and 20,000 dpm/pL for 3H, 22Na and 36C1,respectively. Following the addition of the radiotracers, the cell was operated at current densities of 10, 100 and 300 A/m2. During each interval of constant current, samples of 200-PL volume were extracted from the cold side at equal time periods and transferred to 15-mL volumes of LS cocktail. In addition, the cell was monitored for an extended period of time with no current being passed to measure diffusion rates of the ions and water across the membrane. Periodic samples of the hot-side reservoir were also obtained
ERICW. SCHNE~OW and MARKW. VERLUUJGGE
1402
during the studies. Specific results for the variety of membranes under investigation will be the subject of a future report. Equilibrium studies were also performed to determine the concentrations of mobile species contained within the membranes. This was accomplished by allowing conditioned membranes to equilibrate in a radiotracer solution identical to that of the hot-side reservoir. The membranes were subsequently removed, blotted dry of any excess solution, and equilibrated in 15-mL volumes of LS cocktail before LS and y counting. Redts
and Discussion
Liquid scintillation analyzer eJ%ziency calibration For the triple-label LS analyses, efficiencies were determined by using the quench standards for each of the three nuclides. Spectra of the three sample types, each containing only one of the three tracers, are shown in Fig. 1. Also denoted are the three counting regions selected for triple-label LS analysis. The settings have been shifted lower from the preset values by automatic quench compensation. Resultant efficiency-quench plots for each nuclide in each counting window are reported in Fig. 2. For the dual-label LS analyses, efficiencies were determined in a similar fashion to the triple-label case by using the ‘H and WI quench standards. At nominal quench values for electrolyte samples in this study, the dual-label LS counting efficiency was 53%
for 3H and 95% for 36C1.The interference of each nuclide in the counting region of the other was minor, and never exceeded 3%. Dual-label counting efficiencies for ‘H and ?l were somewhat higher than for the triple-label counting because of the less restrictive energy regions. Assay of hot-side and cold-side triple-label standards The HSS set was counted by both the triple-label LS procedure and dual-label LS/y -ray counting. Results were used to determine which of the two analytical approaches provided the greatest accuracy over the wide range of concentrations evaluated. It can be seen from the spectra in Fig. 1 that virtually all of the intensity from 3H occurs at very low fl energy (Region A). An accurate ‘H assay should therefore be possible without much interference from 22Na or 36C1.The 22Na and 36C1spectra, however, are quite similar, and there will be significant interference whatever regions are selected. Ranges for Regions B and C were chosen for optimal sensitivity to the small differences between the two spectra. Results of the triple-label analysis of the HSS set are reported in Table 3. There is little difficulty in the determination of 3H, regardless of how much 22Na or 36C1is present. However, significant underestimation of 22Na occurs at a variety of ‘H and ‘6C1 concentrations (e.g. Nos 8, 17). Also, significant overestimation of %Cl occurs at low and moderate values of 22Na (e.g. Nos 4, 26). Because there is no other means to
0000
0 g
4000
6 8 z 0 d c g 0
4000
0
200
200
400
600
Energy, keV Fig. 1. Overlaid individual liquid scintillation spectra of ‘H, 22Naand 36C1.Quench-shifted energy regions for the triple-label analyses are also shown. (Spectra have been nonnalii for display purposes.)
6
Radiotracm-ionclrchange
membrane studies
1403
60 l Region
A
(a>
3H
o
o-e-o-q-0 --500
K! n
F
.-9, 0
l Region
A
0 Region
B
l Region
C
-0-m
tF
is 40-
cl
80
60
l Region
A
D Region
B
l Region
C
*-o-o-o-.
40
m-m-B-m-m
Sample Quench, QIP # Fig. 2. Counting e&iency of the LS spectrometer as a function of chmical quench (Quench Indicating Paramcm, QIP No.) for: (a) ‘H, (b) %Ja and (c) %.
ARI 44-10/11-K
ERIC w.
1404
SCHNEIDER
and
Table 3. Analvsis of hot-side standards bv triole-label LS countinn Stock solution added OtL) No.
‘H
1
0 0 0 0 0 0 0 0 0
2 3 4 5 6 7 8 9 10 11 12 I3 14 15 16 17 18 19 20 21 22 23 24 25 26 27
%I
0
0
so
0
WI 0 0 0
300
so
:,
50 50
15 15 15 15 15 15 15 15 15 50 50 50 50 50 50 50 50 50
‘H
0 0 0 0 0 1 0 0 2 0 0
50” 300 0 50 300 0 50 300 0
300 300 300 0 0 0 50 So 50 300 300 300 0 0 0 so 50 50 300 300 300
3:
-1 -2 -2
0 3z 0 SO 300 0 50 300 0 50 300
1 0 I -0 -0 0 -2 -2 -4
Standard deviition
9 -1 -8 20 23 -7 2 0 0 8 5 2 26 40 -I5 4 0 0 5 8 5 37 47 -5
1.2
w.
bUlRUGGE
Table 5. Analysis of hot-side standards counting
0
-2 -30 0 -5 -35 0 -26 -31 0 -7 -39 0 -11 -41 0 -39 -22 0 -3 -33 0 -13 -44 0 -42 -34
14.3
16.7
correct for these deviations, the triple-label LS method for 3H/22Na/36Cl,with the Packard DPM123 software as employed, provides only semi-quantitative results for the range of tracer concentrations of interest in this study, and, more generally, for the characterization of most membrane electrolytes. Table 4. Analysis of hot-side standards before the addition of =Na usiw dual-label LS countina
No.
‘H
1 2 3 4 S 6 7 8 9 10 11 12 13 14 IS 16 17 18 19 20 21 22 23 24 25 26 27
0 0 0 0 0 0 8 0 15 15 15 15 15 15 15 15 I5 50 50 50 50 50 50 so 50 50
WI
“Na
0 0 0 50 50 50 300 300 300 0 0 0 50 50 SO 300
0 50 300 0 50 300 0 50 300 0 SO 300 0 50 300 0
:;
3:
0 0 0 50 50 50 300 300 300
0 50 300 0 50 300 0 50 300
No.
‘H
MC1
*Na
1 2 3 4 5 6 7 8 9 10 11 12 13 14 1s 16
0 0 0 0 0 0 0 0 0 15 15 I5 IS 15 15 1s
0 0 0 50
0 0 0 0 0 0 0 0 0 0 0 0 0 0 :
0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.2 0.3 0.3 0.5 0.1 0.6 0.6 0.1 0.5
0.0 0.0 0.0 -0.1 0.8 0.2 -1.7 -3.7 -1.0 0.0 0.0 0.0 -0.4 -0.2 0.4 3.1
17 18 19 20 21 22 23 24 25 26 27
15 50 50 50 50 50 50 SO 50 50
08
OS 0.4 0.8 0.3 0.8 0.4 0.7 0.7 -0.1 -0.6 -0.2
:.: 0:o 0.0 0.0 0.3 0.8 -0.2 2.3 2.6 1.9
0.3
1.5
z 300 300 300 0 8 SO SO SO 300 300 0 0 0 50 z 300 300 300 Standard deviation
0 0 0 0 0 0 0 0
WI
‘H
‘%I
“Na
0.0 0.2 1.4 0.0 0.2 1.4 0.2 0.2 1.4 0.4 0.6 0.1 1.0 1.0 0.2 0.5 0.5 -0.4 1.2 0.7 0.6 0.8 0.9 -0.8 -0.3 -0.4 -2.0
-0.0 -0.2 7.2 -0.2 0.7 7.6 -3.8 -2.1 8.4 -0.0 0.2 8.2 -0.3 0.2 7.7 2.2 2.3 7.0 0.0 -0.4 ::9 1.3 7.1 0.2 1.5 9.0
0.1 1.2 -3.5 0.1 0.8 -4.1 0.3 1.6 -3.2 0.0 0.4 -3.4 0.0 0.4 -2.8 0.4 0.5 -2.8 -0.0 1.0 -4.1 0.1 0.7 -2.9 0.4 -0.0 -2.5
0.7
3.8
1.8
Results of the dual-label LS and y-ray analyses of the HSS set are reported in Tables 4 and 5. Table 4 shows counting results of the standards before addition of the *%a radiotracer. These values indicate
Table 6. Analysis of cold-side standards countinn
by dual-label LS and y
Stock solution added 01L)
Error @L) ‘H
Error (JIL)
“Na
Standard deviation
Stock solution added (pL)
by dual-label LS and y
Stock solution added OCL)
Error @L)
*Na
0
MARK
No.
IH
: 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
0 0 0 0 0 0 0 0 100 100 100 100 100 100 100 100 100 300 300 300 300 300 300
f: :: 27
300
Error 01 L)
WI
*Na
‘H
WI
=Na
0
100 0 300
-0.0 0.7 1.4 1.5 1.2 2.1 0.3 0.3 2.6 -2.0 0.6 -1.8 2.0 0.3 -0.0 -0.3 -0.3 0.3 -0.6 -2.9 -2.0 -1.5 -2.3 -6.7 -3.2 -3.9 -5.3
-3.1 0.2 0.4 -1.9 1.5 -0.2 -1.4 -2.9 -0.8 -0.7 0.1 1.9 2.0 -1.5 2.1 -2.4 -3.0 -0.9 0.5 -1.2 -4.0 0.9 -1.3 2.9 0.2 0.3 2.2
-0.3 3.4 -2.6 1.4 -0.6 0.2 0.2 -0.0 -1.3 0.8 -0.2 -0.4 -1.8 0.5 -1.7 -0.4 -0.9 1.6 -0.6 1.9 2.7 -0.6 0.9 -0.8 0.2 -2.7 0.0
2.2
1.7
1.4
0
100 100 100 300 300 300 0 0 0 100 100 100 300 300 300 0 0 0 100 100 :$ 300 300
0
loo 300 0 100 300 0 100 300 0 100 300 0 100 300 0 100 300 0 100 300 0 100 300
Standard deviation
Radiotracers-ion-exchange Table 7. Equilibrium uptake study of N&h-l tripk-label radiotracctx
1 2 3
‘H
‘%I
16360 17620 13180
11860 15220 11950
%I
‘H
%I
%Ja
8910 9340 8780
5.1 5.5 4.1
1.0 1.2 1.0
35.2 36.9 34.7
1405
Tabk 8. Initial activity concentrations of each txdiotraw in Nafioo-117 transport studies
17 membranes using Equiv. vol. of clcctrolytc (p L)
Net activity (DPM) No.
membrane studies
Activity amccotfation @PW L) Cell No. 2 3 4 5
that the dual-label approach for simultaneous determination of 3H and “Cl is accurate over a wide concentration range of either tracer. Because y-ray measurements are not alEcted by the presence of ‘H or “Cl, the determination of “Na can be performed accurately and independently of the other tracers. After the uNa was added to the HSS set, y-ray measurements were made and dual-label LS measurements were performed again. Correction factors were then determined to adjust for interferences of the ‘H and WI LS measurements by uNa. The resultant factors, when multiplied by the **Na count rate and added to the measured sample activity level, provide the appropriate corrections to the LS results. Table 5 shows analytical results for the HSS standards using this approach. The correction factors were +0.03 for )H, and - 1.61 for %I. In general, error values are reasonably low across the concentration ranges of each tracer. The only area of concern is the case of samples with high levels of “Na, in which case the “YJl values are overestimated (e.g. Nos 3,6). This error could have been reduced by the selection of a different correction factor for YJl, at the expense of elevating errors at other concentrations. It was decided to employ the correction
‘H
WI
nNa
960 4100 4100 0
3800 20,200 20,200 8100
;o” 0 0
factors as listed and design the experiment to keep the **Na/Yl ratio low enough to avoid the area of concern. As a check of LS/y-ray counting method, the CSS sample set was prepared. The CSS set was similar to the HSS set, but had concentrations of activity at levels expected in the cold reservoir during the iontransport studies. Analytical results for the CSS set are reported in Table 6. The correction factors for =Na obtained from the HSS set were employed in the analysis. Again, reasonable results are obtained for each species over a wide concentration range. Because of the demonstrated accuracy of dual-label LS/y-ray counting method for the HSS and CSS sets, this analytical procedure was selected for the triplelabel radiotracer measurements in all subsequent ion-transport membrane studies. Equilibrium studies Representative results from equilibrium uptake studies of NaGon- 17 membranes are listed in Table 7. Displayed are net disintegration rates of each species as well as the corresponding equivalent volume of electrolyte, which is obtained from the
0
Time, min Fig. 3. Relative concentration of 3H in the cold reservoir as a function of time for four different currents.
EJUCW. ScHNmDER and MARKW. VERBRUOGE
1406
‘j
1.0
C g 0 8
OS .
0.0 0
Time, min Fig. 4. Relative concentration of %I in the cold reservoir as a function of time for four different currents.
ratio of the membrane uptake activity to the activity per microliter of the equilibration solution. Repeatable results have been obtained for all types of membranes studied, and will be reported separately. Count rates for the Na6on membranes were in the same range as a mixture of 5 nL 3H, 14 PL of Yl, and 8 nL %a from the HSS solutions. With reference to Table 5, this tracer range should yield accuracies for all species within about f5%.
Transport studies Transport properties of Nafion- 117 membranes were obtained from a series of four cell studies (Cells 2-5). Initial hot-side concentrations of the tracers in these cells are listed in Table 8. When compared to initial concentrations of ‘H, concentrations of %Cl were elevated to compensate for model predictions (Verbrugge and Hill, 199Oa, b, c) of its low relative transport rate. Likewise, initial concentrations of
0 0
200
400
600
800
1000
1200
1400
14
Time, min Fig. 5. Relative concentration of zzNa in the cold reservoir as a function of time for four different currents.
Radiotracers-ion-exchange
0.0 0
200
400
600
1407
membrane studies
800
1000
1200
1400
16
Time, mln Fig. 6. Relative concentration of 36C1in the cold reservoir as a function of time for four clitIerentcurrents. (Current passage is reversed from the previous cells, with the cold-side electrode being the anode.)
22Na were reduced to compensate for predictions of its high relative transport rate. Cells 2 and 3 employed all three tracers simultaneously. To verify results obtained in the three-tracer studies, Cell 4 employed only ‘H and 36C1,and Cell 5 employed only W. Results from the transport studies of Nafion- 117 are summarized in Figs 3-6. Each plots the ratio of the concentration of the radiotracer in the cold side relative to its concentration in the hot side. All cells were operated for a period of 1440min. Current Regions l-4 labeled on each plot represent current densities of 10, 100, 300 and 0 A/m*, respectively. In each case it is seen that there is a linear increase of activity in the cold reservoir with time at a given current. The slopes of the lines in these plots are used to acquire transport properties. Slight differences in the slopes for equivalent experiments are likely due to differences in the membrane used for each experiment, rather than errors introduced in the radiometric analysis. Other than having different absolute concentrations of the radiotracers, Cells 2 and 3 runs were duplicate experiments. Figures 3, 4 and 5 indicate excellent reproducibility for all species at all currents. Cell 4 was an additional repeat of conditions, but without the *Na radiotracer. Again, Figs 3 and 4 show excellent reproducibility. The agreement between Cell 4 and the previous two runs indicates that the inclusion of the 22Na did not interfere with either the ‘H or %Cl measurements. Results from Cells 2, 3 and 4 all indicated that virtually none of the %Cl radiotracer was transported from the hot side to the cold side when current densities were at or above 100 A/m*. To determine
whether there was a net flow of Cl- in the opposite direction during these conditions, an additional experiment was performed. In this case, only the Wl tracer was employed, and the direction of current passage was reversed so that the cold-side electrode was the anode. Results, shown in Fig. 6, do indeed indicate that the net flow of Cl- is in the opposite direction during high-current conditions. Diffusion rates (Current Region 4) are similar to those measured during the prior three cell runs. Results from these four cell studies indicate that the transport of the three tracers can be tracked simultaneously using the LS/y-ray counting method, and that there are no significant interferences between any of the species. A single experiment can determine the transport rates as well as the diffusion rates of all three species in a given direction as a function of current density. conclusions A triple-label radiotracer method has been developed to characterize ion-exchange membranes for fuel-cell applications. By utilizing ‘HOH for water transport, uNa+ for cation transport, and 3sClfor anion transport, a membrane can be characterized completely and accurately in a single set of measurements. Combinations of the three radiotracers, spanning a wide range of relative concentrations, were used to evaluate various nuclear analytical methods for analysis of electrolyte and membrane samples. The simultaneous determination of all species by triplelabel liquid scintillation counting was found to provide only semi-quantitative results. However, a
Eiuc W. &XNEwsa and
1408
combination of liquid scintittation and y-ray counting provided accurate results over the concentration ranges of interest in this study. To demonstrate the capabilities of this new anatyticat technique, Nation-t t 7 ion-exchange membranes were evaIt&ed for equilibrium conc&trations and transport rates of ail mobile species. Results show that the measurements are reproducible and there are no siunificant interferences between anv of the species. By using the newly developed three-tracer method, we have shown that the transport phenomena of ion-exchange membrane materials can he XCUratetY characterized, which attows US to Predict their utility in fuel-cell applications.
References De Filippis P. (1990) Activity analysis in liquid scintillation coun&g: a. techhical GmpaGtive summary-Part I. Radioact. Radio&m. FdL 22.
Guzman-Garcia A., Verbrugge M. W., Schneider E. W. and Pintauro P. N. (1991)Analysis of radiation-grafted membranes for fuel-cell electrolytes, Research Publication GMR-7225. Presented at tlte Spring Electrochemical Society Meeting, Washington, D.C., May, 1991,Abstract 7%. J. Appi. Electrochem. 22. 204. Herrera A. -and Yeager H. L. (1987) Halide and sulfate ion diffusion in nagon membranes. J. Electrochem. Sot. 134, 2446.
Horrocks D. L. (1982) Triple label DPM determinations by liquid scintillation counting. Trans. Am. Nucl. Sot. 43,39. Ishikawa H., Takiue M. and Aburai T. (1984) Radioassay bv an elEciencv tracing teclmiaue usina a liauid scintill&on counter.-Znr. J. >ppl. R&at. Z&t. 3$‘463. Nkedi-Kizza P.. Jackson R. K. and MacIntvre J. L. (1985) Isotopic triple labeling in miscible displacement studies~ Soil Sci. Sot. Am. J. 49(3), 780 (1985).
MARK
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bRBIlUGGE
Packard Instrument Company (1988a) DPM l-2-3 Option, Operation Manual, Packard/Canberra Industries, Downers Grove, IL. Packard Instrument Company (1988b) Tri-Carb Liquid Scintillation Analyzer Model 22OOCA, Operation Manual, Appendix H, Packard/Canberra Industries, Downers Grove, IL. Pass0 C. (1990) Personal Communication, Packard/ Canberra Industries Marketing Applications Group. Veen H. (1974) Data processing of triple labelled liquid scintillation sam~ks. Inl. J. ad. Radial. Zsot. 2?i. 355. Verbrugge M. W. -&rdHill R. F.-(1988) Experimental and theoretical investigation of pertluorosulfonic acid membranes equilibrated with aqueous sulfuric acid solutions. J. Phys. Chem. 92, 23, 6778.
Verbruaae M. W. and Hill R. F. (199OalIon and solvent transGrt in ion-exchange membranes.‘II. A radiotracer study of the sulfuric-acid, Nalion-117 system. J. Electrochem. Sot. 137. 893.
Verbrugge M. W. .and Hill R. F. (1990b) Transport nhenomena in nerlIuorosulfonic acid membranes during the passage’ of current. J. Electrochem. Sot. 137, 1131. Verbruaae M. W. and Hill R. F. (199Oc) Analysis of prosing perlluorosulfonic acid membranes for fuel-cell electrolytes. J. Electrochem. Sot. 137, 3370. Verbrugge M. W., Hill R. F., Guzman A. and Pintauro P. N. (199Oa)The effect of ionic radii on the absorption and transport characteristics of ion-exchange membranes, Research Publication GMR-6474. Presented at the Spring Electrochemical Society Meeting, Los Angeles, CA, May, 1989, Abstract 626. AZChE J. 36, 1061. Verbrngge M. W., Hill R. F. and Schneider E. W. (199Ob) Composite poly (tetralIuoroetbylene)/perlluorosulfonic acid membranes for fuel-cell applications, Research Publication GMR-7023. Presented at the Annual Meeting of the American Institute of Chemical Enaineers, Chicago, IL, November, 1990, Paper No. 198b. AZChE J. 38, $3. Yeaaer H. L.. O’Dell B. and Twardowski Z. (1982) Transp&t properties of Nation membranes in conc&trated solution environments. J. Elecfrochem. Sot. 129, 85.